Calculate the relative formula masses of the following molecules: O₂, NH₃, C₂H₄, H₂SO₄, HNO₃.

To work out the relative formula mass of a compound, we add up the individual atomic masses from each element. O₂ = 16 x 2 = 32
NH₃ = 14 + (1 x 3) = 17
C₂H₄ = (12 x 2) + (1 x 4) = 28
H₂SO₄ = (1 x 2) + 32 + (16 x 4) = 98
HNO₃ = 1 + 14 + (16 x 3) = 63

What is meant by the term 'moles'?

The term 'moles' refers to a certain amount of an element or compound. If we have one mole of an element, we have 6.02 x 10²³ particles, which is an unimanginably big number (larger than a billion trillion molecules - much easier to say 'one mole').

What is the mass of one mole of nitrogen gas?

One mole of a substance will have the same mass in grams as its formula mass. Nitrogen is diatomic (N₂) so it has an Mr of 28. Therefore one mole of nitrogen gas weighs 28 g.

How many moles are there in 40 g of copper (II) nitrate?

To work out moles we need to divide the mass by the formula mass (Mr). But first, we need to figure out the formula of copper (II) nitrate. The II in brackets indicates that copper has a 2+ charge. Nitrate (NO₃) ions always have a -1 charge (you'll have to memorise this I'm afraid). These charges need to be balanced in the overall molecule, so we need two nitrate ions to balance out the two positive ions from the copper. The formula is: Cu(NO₃)₂. The formula mass is: 63.5 + (2 x 14) + (16 x 6) = 187.5.
Moles = 40 g / 187.5 = 0.213 mol

What is the empirical formula of a molecule containing 13.5 g calcium, 10.8 g oxygen and 0.675 g hydrogen?

First work out the moles of each element by dividing the masses by the molecular mass.
Calcium: 13.5 g / 40 = 0.3375 Oxygen: 10.5 g / 16 = 0.65625 Hydrogen: 0.675 g / 1 = 0.675
Then divide all answers by the smallest answer to find a ratio:
Calcium: 0.3375 / 0.3375 = 1
Oxygen: 0.65652 / 0.3375 = 1.94 - round to 2
Hydrogen: 0.675 / 0.3375 = 2
Empiricial formula = Ca(OH)₂

How many moles are in a 50 cm³ solution of sodium hydroxide with a concentration of 0.8 mol dm^-3

Moles = concentration x volume
First convert volume in cm³ to dm³ by dividing by 1000.
50 / 1000 = 0.05 dm³
Moles = 0.8 x 0.05 = 0.04 mol

What volume does 0.6 moles of neon gas occupy at room temperature?

Every mole of gas at room temperature occupies a volume of 24 dm³.
Volume = moles x 24
Volume = 0.6 x 24 = 14.4 dm³